implement "please update your app" notifications for iOS and android

[biblicabeebli]

• After implementing implement heartbeat feature like on ios beiwe-android#88 we now have the ability to send an arbitrary notification to both apps, along with server knowledge of the app version.
• iOS has always had the ability to receive a notification, so we can implement this with older versions of the iOS app (this isn't much of a problem on iOS tho because we distribute through the app store.
• Implementation shouldn't be too too hard, just need to test app version comparison logic and... never violate it. And automate checking for the most recent version of the app available in the app store. And automate checking the version of the app pushed to the... S3 bucket in use.\
• Due to third parties using custom versions of the app we will need a way to shut this off both per-study and globally.

.... can we also automate sending an email to the system administrator that there is an update to the server code? No? No.